Case hardening



Patented f0... 30, 1934;

UNITED STATES PATENT OFFICE CASE HARDENING No Drawing.

Application October 17, 1932, Serial No. 638,175

12 Claims. (01. 148-15) This invention relates to the case-hardening ofvarious metal articles and more particularly to the case-hardening ofiron or steel articles by treatment in a bath of fused salts containingcyanogen compounds.

A commonmethod of case-hardening various metal articles consists inheating them in' a bath of fused salts containing cyanides, for instancesodium cyanide, or other cyanogen compounds. In one case-hardening bathof this sort large amounts of calcium salt, for instance calciumchloride, are employed and case-hardening properties are imparted to thebath by adding thereto 1% or less of alkali metal cyanide'or othercyanogen compound. These baths are subject to a number of disadvantages,chief among which are the corrosive action of the calcium salt,precipitation of calcium oxide, and the relatively short life of thebath necessitating frequent addition of the cyanogen compound.Furthermore, if substantially more than 1% of cyanogen compound is addedto this bath, it tends to foam and boil with the formation of a heavyscum to such extent that normal operation becomes impossible. Thisabnormal condition persists until the'cyanogen compound content hasdecreased to around 1%. In our co-pending application, Serial Number559,140, we have described a calcium-free case-hardening bath containingconsiderable amounts of barium salt and upwards of of alkali metalcyanide since lo'wer amounts of cy anide do not give rapid enoughpenetration in the presence of barium salts which are relatively moreinactive than the calcium or strontium salts.

The object of this invention is to provide a cal- .cium and/or bariumfree, non-scumming cyanide bath which may be operated for relativelylong periods of time with a cyanogen compound content not greater thanby weight. A further object is to provide such a bath which has arelatively low fusion point and relatively low viscosity at operatingtemperatures.

We have discovered that iron or steel articles may be case-hardened withexcellent results by treating them in a bath of fused salts containingconsiderable quantities of a strontium salt, for instance 25-30%or more,and upwards of 1% of alkali metal cyanide. The case produced is ofexcellent depth and hardness and is characterized by containingrelatively large amounts of both carbon and nitrogen.

In making up a bath in which to carry out our invention we prefer to usean eutectic mixture of strontium and sodium chlorides. This mixturewhich may be made by fusing together substantially equi-molecularweights of the salts, melts at about 560 C. This low melting point makesthis composition especially suitable for case-hardening operations,since it allows operation over a wide range of temperature and insures alow viscosity at the lower temperatures. To this eutectic mixture we add1% by weight or more of cyanogen compound, preferably 2-15% by weight ofalkali metal cyanide. The proper amount of cyanide may vary considerablyand is determined by the desired case-hardening result and the desiredtemperature of operation. For instance, a bath containing 5% by weightof sodium cyanide operated at about 900 C. will produce a hard and deepcase, and the bath is quiescent and substantially non-scumming. If abath containing the above mentioned eutectic and around 15% by weight ofsodium cyanide is operated at 900 C. it also produces a good case butthe bath tends to froth and scum.

The amount of strontium salt used in our casehardening bath may varybetween wide In general, an increase of strontium 'salt contentincreases the case-hardening properties of the bath, cyanide contentremaining constant. Likewise, the case-hardening properties of a bathcontaining a given amount of strontium salt may be increased byincreasing the amount of cyanogen compound. The operating temperaturemay be varied between about 600 C. and 950 C. Increase of temperatureusually increases the activity of the bath and the operating temperatureis adjusted according to the desired results. Wehave obtained excellentresults by operating our bath at around 900 C. with a cyanide content of2-15% by weight.

The tendency to froth and scum in a bath made up according to ourinvention depends on the activity of the bath. Thus, as indicated above,there is some tendency to froth and scum if a bath containing aroundstrontium chloride and 15% sodium cyanide is operated at around 900 C.,while a bath containing around of strontium chloride and 5% of sodiumcyanide operated at 900 C. is quiescent and non-scumming. By varying thethree factors: strontium salt content, cyanide content and temperature,the activity of the bath obviously may be adjusted to any desireddegree, and the bath may be maintained quiescent or not, as the operatordesires. In any case, the tendency to froth and scum in the highlyactive baths is not great and does not seriously interfere with thecase-hardening operation, provided the activity is not substantiallygreater than that of a bath containing 60% of strontium chloride and 15%of sodium cyanide in Example I. The results obtained are given inoperated at 900 C. the following table:

In place of. sodium chloride as used in the above described eutecticmixture, other inert salts, for example potassium chloride, may be usedwith the strontium salt. By inert salt we mean one which has nosubstantial effect on the case-hardening properties of the bath but actsmerely as a.

1st batch 2nd batch 3rd batch 0. s9 0. o. 69 0. s9 0. 79 diluent. Weprefer to use sodium chloride as in- 2nd M4 M1 032 58 Q32 ert saltbecause it is readily available, inexpensive, 43 39 an 0.34 0.27 0.08 0.25 0.06 and gives satisfactory results. While a carbon- 5th (105 M0 ate,for example sodium carbonate, may be used as inert salt, it is not assatisfactory as a chloride because it tends to cause formation ofstrontium carbonate, the presence of which increases the viscosity ofthe bath. It is well recognized in case-hardening practice that a bathof low viscosity is desirable to prevent undue amounts of salt beingremoved along with the treated articles leaving the bath.

Example I Our improved case-hardening bath requires a smaller amount ofcyanide than is necessary in the ordinary cyanide case-hardening bathswhich contain no alkaline earth metal salt. On the other hand, theamount of cyanide added to the bath may be large enough to insurecase-harden= ing action over an extended period of time with: out thenecessity of too frequent additions of cyanide to the bath. The activityof the bath is not too great; that is, emcient case-hardening PPortions, of Strontium P P i action may be obtained without excessivefrothchlonde euttectlc mlxtureg contammg approxl" ing or scumming. Thebath constituents are not matelY eqm'molecular Welghts of the salts werecorrosive to metals. In its preferred form, the fused in separatevessels. To one was added 2% bath has a low melting point which adaptsit for by weight of.sodium cyamdeand to the other 5% use over a widetemperature range. The visa by welght of Sodmm cyamde' F two fusedcosity of the bath is sumciently low to prevent baths were heated toabout 900 and after excessive amounts of salt being removed when theyhad been held at that temperature for about treated articles arewithdrawn therefrom. The

' one hour, rods of 1020 S. A. E. steel were treated case produced hasexcellent depth and contains by immersion in the baths for a period ofone relativel y igh portions of both carbon and nihour. After treatmentthe rods were cooled and trogen An iron or Steel article which has beenWashed free of ajdhermg Salt- The P were case-hardened according to ourinvention may be then each placed in a lathe and successive layersquenched or otherwise heat treated by known 0.004" thick were cuttherefrom. Four succesmeans to obtain a case whose hardness is equal 7sive layers were cut from each rod and each layer s was separatelyanalyzed to determine its carbon and nitrogen contents. The resultsobtained with representative rods are given below:

2 percent NaCN 5 percent NaCN Cut (0.004") Percent 0 Percent N Percent 0Percent N During the operation of the above described baths, sampleswere removed periodically o ver a period of several hours and analyzedto determine 'the sodium cyanide content. These analyses showed that thecyanide was decomposing at the approximate rate of 1% per hour.

\ Example -I I The eutectic mixture described in Example I was preparedand 15% by weight of sodium cyanide was added thereto. The bathtemperature was at about 900 C. when the cyanide was added and thistemperature was maintained throughout the entire operation describedbelow. One hour after the cyanide addition, rods of 1020 S. A. E. steelwere treated in the bath by immersing therein for one hour. At the endof this time another batch of rods was treated in the bath for one hour.Four and one-half hours after the expiration of the second treatment, athird batch of rods was treated in the bath for one hour. The depth andcomposition of the cases obtained were determined by cutting five 0.004"layers to or better than 50 Rockwell units.

The eutectic mixture of strontium and sodium chloride referred to in theappended claims is substantially that which has been described above.

We claim:

l. A method of case-hardening ferrous metal articles comprisingimmersing said articles in a fused salt bath containing a strontiumsalt, alkali 55 metal chloride and alkali metal cyanide, said bath beingsubstantially free from calcium or barium compounds.

2. A method of case-hardening ferrous metal articles comprisingimmersing said articles in a fused salt bath containing strontiumchloride, sodium chloride and sodium cyanide, said bath beingsubstantially free from calcium or barium compounds. l

3. A method of case-hardening ferrous metal articles comprisingimmersing said articles in a fused salt bath containing strontiumchloride, sodium chloride and 2-15% of sodium cyanide, said bath beingsubstantially free from calcium or barium compounds.

4. A method of case-hardening ferrous metal articles comprisingimmersing said articles in a fused salt bath containing 25-60% ofstrontium chloride, 73-25% of sodium chloride and 215% of sodiumcyanide, said bath being substantially free from calcium or bariumcompounds.

5. A method of case-hardening ferrous metal articles comprisingimmersing said articles in a fused salt bath containing sodium cyanideand an eutectic mixture of strontium chloride and sodium chloride, saidbath being substantially free from calcium or barium compounds.

6. A method of case-hardening ferrous metal from the rods and analyzingthem as described articles comprising immersing said articles in a vfused salt bath containing 245% of sodium cyanide and an eutecticmixture or strontium chloride and sodium chloride, said bath beingsubstantially free from calcium or barium compounds.

'I. A salt mixture for case-hardening comprising a strontium salt,alkali metal chloride, alkali metal cyanide and said mixture beingsubstantially free from calcium or barium compounds.

8. A salt mixture for case-hardening comprising strontium chloride,alkali metal chloride, alkali metal cyanide and said mixture beingsubstantially free from calcium or barium com- Pounds.

9. A salt mixture for case-hardening comprising strontium chloride,sodium chloride, sodium cyanide and said mixture being substantiallyfree from calcium or barium compounds.

10. A salt mixture for case-hardening comprising 25-60% of strontiumchloride, sodium chloride, sodium cyanide and said mixture beingsubstantially free from calcium or barium compounds.

11. A salt mixture for case-hardening comprising an eutectic mixture ofstrontium chloride and sodium chloride, 2-15% of sodium cyanide and saidmixture being substantially free from calcium or barium compounds.

12. A salt mixture for case-hardening comprising 25-60% of strontiumchloride, 73-25% of sodium chloride, 2-15% of sodium cyanide and saidmixture being substantially free from calcium or barium compounds.

EARLE ATHERTON HARDING. DONALD AUBREY HOLT.

